A DC/DC converter generates an output DC voltage (one channel converter) or several output DC voltages (multi-channel converter) from an input DC voltage. The general criteria governing the quality of these kinds of converters mainly include their conversion efficiency, the stability of the output voltages and the suppression of high-frequency disturbances. DC/DC converters have a wide range of applications, such as their use in switch mode power supplies.
Multi-channel converters are mostly used in power supplies for computer systems, in particular. In these applications, a power factor controller (PFC) generates an intermediate circuit DC voltage of approx. 400 volts from the mains AC voltage. From this, a downstream multi-channel DC/DC converter generates output voltages of typically 12 volts, 5 volts and 3.3 volts.
The controller in DC/DC converters has the task of keeping its output voltages constant at defined target values despite changing load conditions at the outputs and a possibly erratic input voltage.
The input switching stage of a DC/DC converter generally comprises a pulse width modulator for the purpose of adjusting the power available on the primary side.
As long as only one single output voltage is required (one channel converter), the pulse width of the input switching stage—triggered by a deviation in the actual value of the output voltage from the target value—is changed (e.g. an output voltage smaller than the target value initiates an increase in the pulse width etc.). Depending on the special requirements of the application, different control characteristics can be applied (PI, PID, etc.).
As soon as several output voltages are derived from one input switching stage, it is basically possible to regulate an output voltage in a main output channel, as described above, by controlling the input switching stage (primary control) and to provide a secondary regulation loop for the other output voltages. The problem with this control strategy is that when there is little load at the main output channel, the pulse width on the primary side is narrow and all other channels are likewise limited to a low power consumption.
This method can be especially applied when, owing to the application, one output channel is permanently under a heavier load than all the other channels.
Where this is not a prerequisite of the application, it is preferable if a fixed primary pulse width is chosen und each output channel is regulated by a separate secondary regulation loop. The fixed primary pulse width has to be large enough to provide sufficient power even when there is maximum load.
FIG. 1 shows a block diagram of a power supply of this kind having secondary control loops that are designed as dynamic resonance control loops.
On the input side of the power supply, an upstream PFC 10 converts an input AC voltage into a primary DC voltage that supplies the input switching stage of the multi-channel-DC/DC converter. Variable resonance circuits operate as power throttling elements in the AC voltage region of each output channel. Connected to these are matching transformers and output rectifier networks. The circuit shown in FIG. 1 is published in: “High Power Densities at High Power Levels” by A. Jansen et al. In CIPS 2002, 2nd International Conference on Integrated Power Systems, 11.-12. Jun. 2002, Bremen, DE.
In FIG. 1, the DC input voltage is fed via a full bridge 12 into the primary winding 16 of a main transformer 14. The full bridge 12 switches at a fixed frequency of 700 kHz for example, preferably in ZVS operation (zero-voltage switching). The main transformer 14 delivers identical output voltages to three secondary output windings 18-1, 18-2, 18-3. These are fed via variable impedances 20-1, 20-2, 20-3 and matching transformers 24-1, 24-2, 24-3 into associated rectifier networks 26-1, 26-2, 26-3. The secondary regulation loops are schematically indicated at 22-1, 22-2, 22-3. The variable impedances 20-1, 20-2, 20-3 can be adjusted via controlling currents and connected in series with capacitors. The capacitors ensure that only very low impedance values can be achieved in the vicinity of the resonance of the LC components. The variable impedances 20-1, 20-2, 20-3 can be realized, for example, by a variable inductor, a variable capacitor or an oscillating circuit having a variable resonance frequency or a variable ohmic resistor. A suitable coil arrangement having variable inductance is described, for example, in German Patent Application No. 102 60 246.8 dated 20 Dec. 2002.
FIG. 1 shows a method known in the prior art of controlling a DC/DC converter having a plurality of output channels. The primary side of the main transformer 14 is driven by a fixed pulse width that is large enough to ensure sufficient power output even under maximum load. The secondary output channels are then regulated according to the actual existing load. This control method has the disadvantage that a high reactive power always circulates in the input switching stage. The reactive currents cause energy losses at the ohmic resistors in the input circuit (switching elements, primary winding, strip conductors, input capacitor). In a part-load situation, this goes to reduce the efficiency of the DC/DC converter and, in no-load operation, it results in unnecessarily high energy consumption.
The object of the invention is to provide a DC/DC converter having an input switching stage and a plurality of output channels that operates with optimum efficiency under all load conditions.